1. Field of the Invention
This invention relates generally to a procedure for visually inspecting a given region of a workpiece having drilled holes or other metal surfaces. More particularly, this invention relates to inspection procedures and formulations wherein suitable organic mixtures, such as rubbers or polymerizing materials are admixed with sensitizing agents and magnetizable, or magnetic, particles are used in conjunction with applied magnetic fields of predetermined magnitude and duration to form cast replicas, or impressions, of the metallic surfaces, or regions, of a workpiece; the admixtures being curable, removable, and inspectable for anomalous alignment of the particles against a contrasting background to delineate flaws or potential flaws; such as cracks, scratches, tool marks, dimensional inaccuracies and subsurface flaws that may tend to develop in structurally deficient members.
2. Description of the Prior Art
The prior art has seen the development of many forms of nondestructive testing. One such method has been magnetic particle inspection. The most pertinent art of which we are aware is U.S. Pat. No. 3,862,047, entitled "Method and Composition for Detecting Flaws in Metallic Surfaces, Holes and Otherwise", inventors Henry J. Weltman, Mark T. Carrol, John E. Halkias, William T. Kaarlela, and Jack D. Reynolds, assigned to the General Dynamics Corporation, Fort Worth, Texas; and the descriptive matter of that patent is incorporated herein by reference for details omitted herefrom in the interests of brevity.
U.S. Pat. No. 3,862,047 contains an excellent discussion of the development and other pertinent references. As discussed therein, magnetic particle inspection of critical steel parts has been an accepted nondestructive testing practice within aerospace industries for many years. This is evidenced by the number of patents cited; namely, U.S. Pat. Nos. 2,601,212; 2,744,040; 2,791,561; 3,087,832; 3,345,564; and Noll Chemistry and Technology of Silicones, Academic Press, New York, New York, 1968, pages 397-9. Additional patents that are pertinent are British Pat. No. 933,701, U.S. Pat. Nos. 2,848,748; 3,445,759.
Of the methods disclosed, the most commonly known method previously used consisted of coating the area to be inspected with a suspension of magnetic iron oxide suspended in a volatile petroleum oxide type solvent or introducing the magnetic particles to the proposed area as a dry powder. After coating, and regardless of whether the magnetic particles were disseminated by the liquid or the powder, the area to be tested was magnetized in a known direction. The magnetic particles, upon application of the magnetic field, migrate to the secondary magnetic field created by any flaws or discrepancies that are present. The magnetic particles become aligned over any defect because of an interruption of the magnetic lines of flux in the component or area affected, thus disclosing the presence of the defect or irregularity. This process is generally not applicable to inspection of internal surfaces of holes and is necessarily limited to use of readily accessible surfaces.
Normal magnetic particle inspection provides visual indication of surface defects because the particles concentrate at the defective area. It is possible to enhance visibility by coating the magnetic particles with a dye, known in the industry as Magnaflux. Another process is known as Magnaglow in which the magnetic particles are coated with a flourescent dye and detection is accomplished under ultra-violet light. These methods did not provide a permanent record of the inspection. Other methods employed film forming constituents in which the magnetic particles were oriented in the magnetic field and a relatively rapidly drying film was used to preserve the record of the orientation of magnetic particles, although in a fragile type film. Moreover, these fragile films did not lend themselves to application in difficultly accessible areas, such as around holes or the like.
A further variation was employed in which a type of plastic, such as polyvinyl alcohol dispersed in a volatile solvent, was cast on top of the area to be inspected. When the solvent evaporated, the resultant layer was reinforced, removed, and the orientation of the magnetic particles detectable. These types of castings, while permanent, are, as compared to the method hereinafter disclosed, relatively difficult to remove. A further disadvantage is their limitation in use to top horizontal planes and lack of applicability in inspection of deep or threaded holes. Additionally, the volatile solvent is an undesirable feature.
The aforementioned U.S. Pat. No. 3,862,047 disclosed a method and composition for overcoming the disadvantages of the prior art. Specifically, the method and composition of that patent allowed inspecting any region of any part. Small flaws that require magnification for their detection could be delineated. The method allowed access to difficultly inspected positions such as interiorly of cavities, holes, and the like. Even that improved method had, however, some disadvantages. For example, the permanent record that was formed and removed from the region of the workpiece tended to have a portion of its diluent volatile so as to vaporize and cause shrinkage. Consequently, the mold was not as large as initially formed. Moreover, the constituents of the composition were oxidizable at temperatures below 100.degree. F, and even at room temperatures, or less. Consequently, the materials became rancid, or were oxidized, and created an odor problem. In addition, constituents of the diluent tended to be exuded or secreted, as a liquid to form droplets on the exterior of the mold, or casting.
Thus, it can be seen that the prior art, even the improved method and composition in accordance with U.S. Pat. No. 3,862,047, did not provide a completely satisfactory long term solution.